1. Field of the Invention
The present invention relates generally to laser systems, and more particularly, to improved laser mode stabilization.
2. Related Art
Developers of information storage devices continue to seek increased storage capacity. As part of this development, holographic memory systems have been suggested as alternatives to conventional memory devices. Holographic memory systems may be designed to record data one bit of information (i.e., bit-wise data storage). See McLeod et al. “Micro-Holographic Multi-Layer Optical Disk Data Storage,” International Symposium on Optical Memory and Optical Data Storage (July 2005). Holographic memory systems may also be designed to record an array of data that may be a 1-dimensional linear array (i.e., a 1×N array, where N is the number linear data bits), or a 2-dimension array commonly referred to as a “page-wise” memory systems. Page-wise memory systems may involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the system stores, in three dimensions, the pages of data holographically as patterns of varying refractive index imprinted into a storage medium. See Psaltis et al., “Holographic Memories,” Scientific American, November 1995, where holographic systems are discussed generally, including page-wise memory systems.
In a holographic data storage system, information is recorded by making changes to the physical (e.g., optical) and chemical characteristics of the holographic storage medium. These changes in the holographic medium take place in response to the local intensity of the recording light. That intensity is modulated by the interference between a data-bearing beam (the data beam) and a non-data-bearing beam (the reference beam). The pattern created by the interference of the data beam and the reference beam forms a hologram which may then be recorded or written in the holographic medium. If the data-bearing beam is encoded by passing the data beam through, for example, a spatial light modulator (SLM), the hologram(s) may be recorded or written in the holographic medium as holographic data.
The formation of the hologram may be a function of the relative amplitudes, phase, coherence, and polarization states of the data and reference beams. It may also depend on the relative wavelength of the data and reference beams, as well as the three dimensional geometry at which the data and reference beams are projected towards the storage medium. The holographically-stored data may be retrieved by performing a data read operation, also referred to as a data reconstruction operation (collectively referred to herein as a “read” operation). The read operation may be performed by projecting a reconstruction or probe beam into the storage medium at the same angle, wavelength, phase, position, etc., as the reference beam used to record or write the data, or compensated equivalents thereof. The hologram and the reconstruction beam interact to reconstruct the data beam which may then be detected by using a sensor, such as a photo-detector, sensor array, camera, etc. The detected reconstructed data may then be processed for delivery to, for example, an output device.
Because the recording and reading of the hologram is a function of the wavelengths, amplitudes, phase, coherence, and polarization states of the light beams used, errors in these light beams may result in errors in the recording and reading of the holographic data. For example, it may be desired that the light beams include only a single longitudinal mode (i.e., a single dominant wavelength), as the presence of multiple longitudinal modes (i.e., multiple wavelengths with significant power) within a light beam may result in reduced hologram strength and subsequently errors when recording data to and/or reading data from a holographic storage medium. The presence of multiple modes in a light beam (e.g., a laser) is typically characterized by the Side Mode Suppression Ratio (SMSR). This is a ratio of the power in the primary wavelength peak to the power in the second most prevalent wavelength peak. A laser operating in single mode has a much higher value of SMSR than one operating in multimode. For example, if the single mode requirement was that the secondary wavelength had a peak power of <1% of the primary lasing wavelength, the SMSR would need to be >20 dB to meet this requirement.
Thus, there may be a need for improved methods and systems for determining whether or not multiple modes are or may be present within a light beam and for adjusting the light source so that is in single mode operation.